This invention deals with a motor-compressor unit that runs on compressed air, and more particularly, using a piston travel control device that stops the piston at top dead centre for a period of time and engine rotation, together with a variable-volume work-producing active chamber, an integrated (or not) compression device, and a device for recovering ambient thermal energy. It can operate in mono-energy mode (fuel or compressed air) or in bi-energy mode (fuel and compressed air, simultaneously or successively).